Beyond cell-cell adhesion: Emerging roles of the tight junction scaffold ZO-2

Two Novel Pathogenic Variants of TJP2 Gene and the Underlying Molecular Mechanisms in Progressive Familial Intrahepatic Cholestasis Type 4 Patients

Progressive familial intrahepatic cholestasis (PFIC) is an autosomal recessive inherited disease that accounts for 10%-15% childhood cholestasis and could lead to infant disability or death. There are three well-established types of PFIC (1-3), caused by mutations in the ATP8B1, ABCB11, and ABCB4 genes. Biallelic pathogenic variants in the tight junction protein 2 gene (TJP2) were newly reported as a cause for PFIC type 4; however, only a limited number of patients and undisputable variants have been reported for TJP2, and the underlying mechanism for PFIC 4 remains poorly understood. To explore the diagnostic yield of TJP2 analysis in suspected PFIC patients negative for the PFIC1-3 mutation, we designed a multiplex polymerase chain reaction-based next-generation sequencing method to analyze TJP2 gene variants in 267 PFIC patients and identified biallelic rare variants in three patients, including three known pathogenic variants and two novel variants in three patients. By using CRISPR-cas9 technology, we demonstrated that TJP2 c.1202A > G was pathogenic at least partially by increasing the expression and nuclear localization of TJP2 protein. With the minigene assay, we showed that TJP2 c.2668-11A > G was a new pathogenic variant by inducing abnormal splicing of TJP2 gene and translation of prematurely truncated TJP2 protein. Furthermore, knockdown of TJP2 protein by siRNA technology led to inhibition of cell proliferation, induction of apoptosis, dispersed F-actin, and disordered microfilaments in LO2 and HepG2celles. Global gene expression profiling of TJP2 knockdown LO2 cells and HepG2 cells identified the dysregulated genes involved in the regulation of actin cytoskeleton. Microtubule cytoskeleton genes were significantly downregulated in TJP2 knockdown cells. The results of this study demonstrate that TJP2 c.1202A > G and TJP2 c.2668-11A > G are two novel pathogenic variants and the cytoskeleton-related functions and pathways might be potential molecular pathogenesis for PFIC.

Senescence suppressed proliferation of host hepatocytes is precondition for liver repopulation

In the fumarylacetoacetate hydrolase deficient (Fah^-/-) mouse, massive liver repopulation can be easily obtained after transplanted hepatocytes. Understanding the mechanisms of complete liver repopulation in Fah^-/- mice will be useful for future clinical application. Here, we found that the endogenous hepatocytes in liver of Fah^-/- mice undertook senescence during the time of tyrosinemia symptoms. Increase of senescent hepatocytes in Fah^-/- mice provided proliferative advantage to the transplanted hepatocytes. Importantly, senescent hepatocytes upregulated the expression of extracellular matrix enzyme, contributing to degradation of extracellular matrix components and weakness of cell adhesion and connection. The liver exhibiting a loose architecture provided the space for the engraftment and expansion of transplanted hepatocytes. These findings underscore the underlying mechanisms of completed liver repopulation in Fah^-/- mice. Senescence followed by loose hepatic parenchyma is a preconditioning for liver repopulation, which would be a promising strategy to achieve therapeutic liver repopulation in clinical settings.

Molecular Regulation of Sprouting Angiogenesis

The term angiogenesis arose in the 18th century. Several studies over the next 100 years laid the groundwork for initial studies performed by the Folkman laboratory, which were at first met with some opposition. Once overcome, the angiogenesis field has flourished due to studies on tumor angiogenesis and various developmental models that can be genetically manipulated, including mice and zebrafish. In addition, new discoveries have been aided by the ability to isolate primary endothelial cells, which has allowed dissection of various steps within angiogenesis. This review will summarize the molecular events that control angiogenesis downstream of biochemical factors such as growth factors, cytokines, chemokines, hypoxia-inducible factors (HIFs), and lipids. These and other stimuli have been linked to regulation of junctional molecules and cell surface receptors. In addition, the contribution of cytoskeletal elements and regulatory proteins has revealed an intricate role for mobilization of actin, microtubules, and intermediate filaments in response to cues that activate the endothelium. Activating stimuli also affect various focal adhesion proteins, scaffold proteins, intracellular kinases, and second messengers. Finally, metalloproteinases, which facilitate matrix degradation and the formation of new blood vessels, are discussed, along with our knowledge of crosstalk between the various subclasses of these molecules throughout the text. Compr Physiol 8:153-235, 2018.

PARP‑1 may be involved in hydroquinone‑induced apoptosis by poly ADP‑ribosylation of ZO‑2

Hydroquinone (HQ), a major reactive metabolite of benzene, contributes to benzene-induced leukemia. The molecular mechanisms that underlie this activity remain to be elucidated. Poly ADP-ribosylation (PARylation) is a type of reversible posttranslational modification that is performed by enzymes in the PAR polymerase (PARP) family and mediates different biological processes, including apoptosis. Zona occludens 2 (ZO-2) is a tight junction scaffold protein, which is involved in cell proliferation and apoptosis. The present study investigated the activity and mechanisms regulated by PARP-1 during HQ-induced apoptosis using TK6 lymphoblastoid cells and PARP-1-silenced TK6 cells. The results revealed that exposure to 10 µM HQ for 72 h induced apoptosis in TK6 cells and that apoptosis was attenuated in PARP-1-silenced TK6 cells. In cells treated with HQ, inhibition of PARP-1 increased the expression of B cell leukemia/lymphoma 2 (Bcl-2), increased ATP production and reduced reactive oxygen species (ROS) production relative to the levels observed in cells treated with HQ alone. Co-localization of ZO-2 and PAR (or PARP-1 protein) was determined using immunofluorescence confocal microscopy. The findings of the present study revealed that ZO-2 was PARylated via an interaction with PARP-1, which was consistent with an analysis of protein expression that was performed using western blot analysis, which determined that ZO-2 protein expression was upregulated in HQ-treated control cells and downregulated in HQ-treated PARP-1-silenced TK6 cells. These findings indicated that prolonged exposure to a low dose of HQ induced TK6 cells to undergo apoptosis, whereas inhibiting PARP-1 attenuates cellular apoptosis by activating Bcl-2 and energy-saving processes and reducing ROS. The present study determined that PARP-1 was involved in HQ-induced apoptosis by PARylation of ZO-2.

SUMOylation regulates the intracellular fate of ZO-2

The zonula occludens (ZO)-2 protein links tight junctional transmembrane proteins to the actin cytoskeleton and associates with splicing and transcription factors in the nucleus. Multiple posttranslational modifications control the intracellular distribution of ZO-2. Here, we report that ZO-2 is a target of the SUMOylation machinery and provide evidence on how this modification may affect its cellular distribution and function. We show that ZO-2 associates with the E2 SUMO-conjugating enzyme Ubc9 and with SUMO-deconjugating proteases SENP1 and SENP3. In line with this, modification of ZO-2 by endogenous SUMO1 was detectable. Ubc9 fusion-directed SUMOylation confirmed SUMOylation of ZO-2 and was inhibited in the presence of SENP1 but not by an enzymatic-dead SENP1 protein. Moreover, lysine 730 in human ZO-2 was identified as a potential modification site. Mutation of this site to arginine resulted in prolonged nuclear localization of ZO-2 in nuclear recruitment assays. In contrast, a construct mimicking constitutive SUMOylation of ZO-2 (SUMO1ΔGG-ZO-2) was preferentially localized in the cytoplasm. Based on previous findings the differential localization of these ZO-2 constructs may affect glycogen-synthase-kinase-3β (GSK3β) activity and β-catenin/TCF-4-mediated transcription. In this context we observed that ZO-2 directly binds to GSK3β and SUMO1ΔGG-ZO-2 modulates its kinase activity. Moreover, we show that ZO-2 forms a complex with β-catenin. Wild-type ZO-2 and ZO-2-K730R inhibited transcriptional activity in reporter gene assays, whereas the cytosolic SUMO1ΔGG-ZO-2 did not. From these data we conclude that SUMOylation affects the intracellular localization of ZO-2 and its regulatory role on GSK3β and β-catenin signaling activity.

PHF11 expression and cellular distribution is regulated by the Toll-Like Receptor 3 Ligand Polyinosinic:Polycytidylic Acid in HaCaT keratinocytes

Background

Inflammatory skin diseases such as atopic dermatitis and psoriasis represent a complex interaction between the skin and infiltrating immune cells, resulting in damage to the skin barrier and increased inflammation. Polymorphisms in PHF11 have been associated with dermatitis and allergy and PHF11 regulates the transcription of T-cell cytokines as well as class switching to IgE in activated B-cells. The importance of skin barrier homeostasis in the context of inflammatory skin diseases, together with reports identifying PHF11 as an interferon-induced gene, have led us to examine its role in the innate immune response of keratinocytes.

Results

We developed a cell culture model that allowed us to analyze the effects of the double-stranded RNA analogue poly(I:C) on a confluent cell monolayer immediately after a 24-h treatment, as well as three days after withdrawal of treatment. Immediately after treatment with poly(I:C), PHF11, IL8, and interferon-dependent ISG15 RNA expression was increased. This was accompanied by nuclear localization of PHF11 as well the tight junction protein claudin-1. Knock-down of PHF11 resulted in increased interleukin-8 expression and secretion immediately following treatment with poly(I:C), as well as changes in the cellular distribution of membrane-bound and increased nuclear claudin-1 that was observed up to 3 days after the withdrawal of poly(I:C). This was associated with lower cell density and a decrease in the number of cells in the G1 phase of the cell cycle.

Conclusions

In addition to a role for PHF11 in lymphocyte gene expression, we have now shown that PHF11 was part of the keratinocyte innate immune response by poly(I:C). As knock-down of PHF11 was associated with increased expression of the pro-inflammatory chemokine IL-8 and changes in the cellular distribution of claudin-1, a change normally associated with increased proliferation and migration, we suggest that PHF11 may contribute to epidermal recovery following infection or other damage.

The Mammalian Blood-Testis Barrier: Its Biology and Regulation

Spermatogenesis is the cellular process by which spermatogonia develop into mature spermatids within seminiferous tubules, the functional unit of the mammalian testis, under the structural and nutritional support of Sertoli cells and the precise regulation of endocrine factors. As germ cells develop, they traverse the seminiferous epithelium, a process that involves restructuring of Sertoli-germ cell junctions, as well as Sertoli-Sertoli cell junctions at the blood-testis barrier. The blood-testis barrier, one of the tightest tissue barriers in the mammalian body, divides the seminiferous epithelium into 2 compartments, basal and adluminal. The blood-testis barrier is different from most other tissue barriers in that it is not only comprised of tight junctions. Instead, tight junctions coexist and cofunction with ectoplasmic specializations, desmosomes, and gap junctions to create a unique microenvironment for the completion of meiosis and the subsequent development of spermatids into spermatozoa via spermiogenesis. Studies from the past decade or so have identified the key structural, scaffolding, and signaling proteins of the blood-testis barrier. More recent studies have defined the regulatory mechanisms that underlie blood-testis barrier function. We review here the biology and regulation of the mammalian blood-testis barrier and highlight research areas that should be expanded in future studies.

Pathological implications of cadherin zonation in mouse liver

Both acute and chronic liver diseases are associated with ample re-modeling of the liver parenchyma leading to functional impairment, which is thus obviously the cause or the consequence of the disruption of the epithelial integrity. It was, therefore, the aim of this study to investigate the distribution of the adherens junction components E- and N-cadherin, which are important determinants of tissue cohesion. E-cadherin was expressed in periportal but not in perivenous hepatocytes. In contrast, N-cadherin was more enriched towards the perivenous hepatocytes. In agreement, β-catenin, which links both cadherins via α-catenin to the actin cytoskeleton, was expressed ubiquitously. This zonal expression of cadherins was preserved in acute liver injury after treatment with acetaminophen or partial hepatectomy, but disrupted in chronic liver damage like in non-alcoholic steatohepatitis (NASH) or α1-antitrypsin deficiency. Hepatocyte proliferation during acetaminophen-induced liver damage was predominant at the boundary between the damaged perivenous and the intact periportal parenchyma indicating a minor contribution of periportal hepatocytes to liver regeneration. In NASH livers, an oval cell reaction was observed pointing to massive tissue damage coinciding with the gross impairment of hepatocyte proliferation. In the liver parenchyma, metabolic functions are distributed heterogeneously. For example, the expression of phosphoenolpyruvate carboxykinase and E-cadherin overlapped in periportal hepatocytes. Thus, during liver regeneration after acute damage, the intact periportal parenchyma might sustain essential metabolic support like glucose supply or ammonia detoxification. However, disruption of epithelial integrity during chronic challenges may increase susceptibility to metabolic liver diseases such as NASH or vice versa. This might suggest the regulatory integration of tissue cohesion and metabolic functions in the liver.

In vitro models of the blood-brain barrier for the study of drug delivery to the brain

The most important obstacle to the drug delivery into the brain is the presence of the blood-brain barrier, which limits the traffic of substances between the blood and the nervous tissue. Therefore, adequate in vitro models need to be developed in order to characterize the penetration properties of drug candidates into the central nervous system. This review article summarizes the presently used and the most promising in vitro BBB models based on the culture of brain endothelial cells. Robust models can be obtained using primary porcine brain endothelial cells and rodent coculture models, which have low paracellular permeability and express functional efflux transporters, showing good correlation of drug penetration data with in vivo results. Models mimicking the in vivo anatomophysiological complexity of the BBB are also available, including triple coculture (culture of brain endothelial cells in the presence of pericytes and astrocytes), dynamic, and microfluidic models; however, these are not suitable for rapid, high throughput studies. Potent human cell lines would be needed for easily available and reproducible models which avoid interspecies differences.